1. Field of the Invention
This invention relates to a method and an apparatus for perforating, on a heat sensitive film, a pattern in dot matrix form corresponding to an original image, by using a thermal head. The heat sensitive film is substantially made only of a thermoplastic resin film.
2. Description of the Related Art
A heat sensitive stencil perforating system is well-known. In such a system, a thermoplastic resin film of approximately 1.5 .mu.m to 2 .mu.m in thickness and a porous base support, such as Japanese paper, are bonded to form a heat sensitive stencil. A thermal head is used to perforate the stencil with a pattern of an original image in dot matrix form. For instance, Japanese Patent Laid-open Publication No. Hei 2-067,133 discloses a thermal head for use with the foregoing stencil perforating system, i.e. the thermal head has heating resistors whose pitch is shorter in a stencil feeding direction (i.e. a sub-scanning direction) than a pitch in a direction orthogonal to the stencil feeding direction (i.e. a main scanning direction). Further, Japanese Patent Laid-open Publication No. Hei 4-045,936 proposes a thermal head including heating resistors whose cross section is a shape other than rectangular.
Referring to FIG. 14 of the accompanying drawings, the stencil perforating system of the prior art uses a heat sensitive stencil 1 made of a thermoplastic resin film 1a and a porous base support 1b, both of which are bonded. The heat sensitive stencil 1 is conveyed to a space between a platen roller 3 and a thermal head 4, so that the thermophetic resin film 1a is directly perforated by heating resistors 5 of the thermal head 4.
The foregoing stencil perforating system is, however, prone to the following problems depending upon characteristics of the heat sensitive stencil. When fibers are sparse or coarse in the porous base support, the heat sensitive stencil cannot be brought into close contact with the heating resistors of the thermal head, which might result in erratic perforations or non-perforation of the stencil. Further, when the fibers are close in the base support, or when they are lumpy, the heat sensitive stencil is also subjected to erratic perforation. This is because heat applied by the thermal head is absorbed by the fibers, or melted remains of the heat sensitive film stick to the fibers. Further, when fibers in the base support are too thin, the thermoplastic resin film tends to stick closely to the heating resistors, which undesirably enlarges the perforations. As shown in FIG. 15, sometimes perforations 102 are not formed at positions where they should be, they tend to have different shapes or sizes, or they are so large that they link with adjacent perforations.
Therefore, when the foregoing stencils are used in the printing operation, they would produce images which have faded portions, images which have non-inked white spots in solid portions, images which are partially printed, or images which have non-inked white portions in the shape of fibers of the base support, or would cause ink on a previously printed sheet to be transferred to the back surface of the next printed sheet (called "back transfer").
Further, ink cannot pass through the stencil at portions where fibers are lumpy in the base support or where an amount of adhesive is excessive, so that such a stencil would also produce images similar to those described above.
To overcome the foregoing problems, it has been proposed to use a heat sensitive stencil made substantially only of a thermoplastic resin film of 1.5 .mu.m to 2 .mu.m in thickness without any base support such as Japanese paper. However, when such a heat sensitive stencil is perforated by a conventional stencil perforating system, a space c (FIG. 16) between two adjacent perforations 122 is too small, which would reduce the plate wear of the heat sensitive stencil 121.
The heat sensitive stencils made of substantially only a thermoplastic resin film are classified into a heat sensitive stencil containing only the thermoplastic resin film, a thermoplastic resin film containing a minute amount of an antistatic agent, and a thermoplastic resin film which has at least one overcoat layer at least on either a front or rear surface thereof.
For instance, when an original image 100 has a number of horizontal lines as shown in FIG. 17, a stencil 101 is accordingly perforated as shown in FIG. 18. FIG. 19 is an enlarged view of the horizontal line area B of the stencil 101 (shown in FIG. 18), and shows that perforations 102 are arranged orderly thereon. Then, the perforated stencil 101 is wound around an outer surface of a cylindrical printing drum 103 with the leading edge thereof held by a clamp 104. Thereafter, print sheets are successively pressed toward the printing drum 103 by a press roller so that the image perforated on the stencil 101 will be printed on the print sheets. In this state, the press roller applies tension to the stencil 101 in the direction X (FIG. 18). Therefore, the stencil 101 is accordingly stretched together with the perforations 102 thereon as shown in FIG. 20. This means that some printed horizontal lines will become unnecessarily thick. This phenomenon is often considerable in a horizontal line area B and a central area D as shown in FIG. 18.
Further, as the printing operation is repeated, the stencil 101 may become broken around an area C where the stencil was perforated to indicate a circle (shown in FIG. 18), and will be peeled off from the printing drum 103 at the area inside the circle. In FIG. 18, the printing drum 103 is shown in the expanded flat form.